French Pat. No. 2 473 248 for example discloses plasma torches in which each of the electrodes, generally made of metal, is in one piece and is shaped to include a tubular intermediate part and two end parts whereby the electrode maybe connected to other pieces of the torch, said electrodes being disposed within sealed chambers in which a cooling fluid circulates.
In electrodes of this type, the tubular intermediate part is normally of simply shape and serves to catch the arc feed, whilst the end parts are complex in shape but do not normally come in contact with the arc feet. These electrodes are thus normally considered expendable pieces of which only the tubular intermediate part erodes.
Two methods are essentially known for making such one-piece electrodes:
the first method consists in making, then in machining, a monolithic electrode blank. This method makes it possible to obtain thin electrodes wherein the electrode wall has a thickness of some millimeters or less. Consequently, these electrodes wear rapidly and must be replaced frequently, thereby giving rise to high costs of use and limitations of the performances of the torches;
the second method of making one-piece electrodes consists of producing and machining blanks of electrode parts, then in welding said pieces together by electron bombardment. This second method makes it possible to obtain thicker electrodes than those obtained by the first method. However, owing to the mode of welding, it is hardly possible to obtain an electrode thickness greater than ten millimeters, with the result that the electrodes thus produced must also be frequently replaced. In addition to the fact that welding by electron bombardment does not allow connection of thick pieces, it risks causing surface irregularities and differences in the welding zone, leading to rapid breakdown of said electrodes. Furthermore, if, an electrode obtained by electron bombardment welding is sought to be serviced by replacing the eroded part of the electrode and welding a new electrode component to the remaining portions of the electrode, the electrode thus obtained will likely exhibit inferior alignment of its components in comparison to a new electrode. This results in deterioration in the performances of the torch.
In addition, whatever the known method of making the electrodes, taking into account the fact that an electrode of appropriate metal must be associated with the plasmagenic gas used, the electrodes must be entirely replaced if it is desired to employ the torch with different plasmagenic gases.